A double-blind, placebo-controlled study using the antiandrogen compound flutamide in 30 patients with benign enlargement of the prostate is reported. The potency of the compound was indicated by the large percentage of patients suffering from gynecomastia or nipple pain. Flow rate recordings are probably the most reliable and useful examination in this type of investigation, and statistical analysis of the results showed evidence of significant improvement in patients receiving flutamide. No evidence of an effect as compared to the placebo was found when the residual urine, prostate size or histological changes in prostatic biopsies were examined. Subjective effects, when carefully analyzed, provided some evidence of a preference for the flutamide group, especially in the early weeks of treatment, but the fallaciousness of subjective observations is stressed. The various problems associated with the choice and measurement of parameters to be used in this type of investigation are discussed, and the absolute necessity of proper controls and statistical analysis in such a clinical study is illustrated.